When a Category Menu is used (instead of using a Category Module assigned to a Menu), I can see some general formating issues: for instance, a 3 column listing is formated as width:33.3% in desktop and will be formated as width:33.3% in mobil version too. The result will be 1 column using 1/3 of the screen.
Is there an easy way to avoid this effect?
Should I convert Category Menus to Category Modules to be able to format desktop/mobil separately?
Thank you.

Hello,

from your description it looks like the layout is based on tables. This would mean that also on mobile, the screen width will be distributed for each table column in the same way. To have a single column with 100% width the table could require custom CSS for mobile, but i have to look at that page to better understand and help you.

Thanks